Acute heart transplant (HTx) rejection is typically averted by available immunosuppressants, which control recipient CD4+ and CD8+ T cell responses to alloantigens presented by the HTx. Unfortunately, these drugs are unable to prevent chronic rejection, -an immune-driven process of pathogenic fibrotic remodeling of the myocardium and vasculature. Chronic rejection is a significant clinical PROBLEM and leads to the dysfunction and loss the majority of HTx in a little over ten years post transplantation. Approaches regulating chronic rejection-promoting immune responses AND shaping HTx repair are needed to solve this problem. We have discovered a subset of Treg that express ST2, the receptor for interleukin(IL)-33. Our preliminary data suggest that ST2+Treg respond to IL-33 with mechanisms controlling local inflammation and supporting tissue repair. IL-33 is expressed by cells of the HTx and is released in a functional form during tissue damage. In early studies we revealed that IL-33 administration post MHC-mismatched heart transplantation expanded Treg that tripled graft survival in the absence of immunosuppression. Our recent comparisons of ST2+ and ST2- Treg found that, while both are able to control T cell mediated acute HTx rejection, IL-33 stimulation of ST2+Treg is critical for prevention of chronic rejection-associated immune infiltration and myocardial fibrosis. These data support our CENTRAL HYPOTHESIS that ST2+Treg are important for preventing chronic HTx rejection due, not only to their capacity to suppress AlloAg-reactive T cells, but also their ability to facilitate tissue repair and regulat myeloid cells in response to IL-33. The OBJECTIVE of this application is to identify exploitable mechanisms by which ST2+Treg and IL-33 control inflammation and mediate cardiac tissue repair after transplantation. To that end, we will perform HTx and cardiac injury studies utilizing transgenic mice that allow specific gene targeting in Treg and donor and recipient mice lacking IL-33. Our data also suggest that chronic HTx rejection may arises as a result of IL-12 cytokines that suppress ST2+Treg and favor IL-33 stimulation of deleterious CD8+T cell responses. Thus, we will also take advantage of HTx models where ST2 is absent on CD8+ T cells or Treg and IL-33 and IL-12 cytokines targeted in host cells. The requirement of Treg suppression of CD4+ and CD8+ T cell responses for maintenance of peripheral tolerance and induction of Tx tolerance is well established. The concept that local or systemic IL-33 stimulates the reparative capacity of ST2+Treg during tissue injury is novel and untested. The proposed studies are INNOVATIVE because they offer a new way of thinking about the role Treg play in coordinating the cytokine networks controlling HTx outcomes. These studies are also SIGNIFICANT, as they will identify targetable mechanisms controlling the reparative capacity of ST2+Treg in transplantation.